The development of any organ depends on the division, movement, and differentiation of a primordia of undifferentiated cells. The alterations in gene expression which contribute to the differentiation of the two interacting photosynthetic cell types in maize leaves (mesophyll and bundle sheath) are most dramatic among the enzymes which interact in C4 carbon fixation. Each C4 enzyme is specific to one cell type, and each is either plastid- or cytoplasm- localized. Several of the C4 enzymes are present in isozymic forms which are time- and tissue-specific in their appearance. This proposal seeks to characterize the C4 enzymes phosphoenolpyruvate carboxylase, NADP-malate dehydrogenase, NADP-malic enzyme, pyruvate-Pi dikinase, and a spartate aminotransferase with respect to (1) the source and function of isozymic forms expressed in development, (2) the importance of bundle sheath-mesophyll interactions in cell-specific expression, and (3) the influence of plastid state upon the expression of these nucleus-encoded enzymes. Protein and mRNA expression will be measured with antisera and cDNA probes, respectively. The study of bundle sheath-mesophyll interactions will be aided by the use of certain maize developmental mutants and by cell-separation methods. Other mutants will be helpful in distinguishing plastid effects on C4 expression. Information gained from the study of cell-specific C4 expression during leaf development should be generally applicable to the understanding of gene expression during any programmed cell differentiation.